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  • 1
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    Reflection of Solar Wind Hydrogen from the Lunar Surface (2013)
    Wiley
    Details
    Publication Date: 2013-01-29
    Description: . [1]  The solar wind continuously flows out from the Sun and directly interacts with the surfaces of dust and airless planetary bodies throughout the solar system. A significant fraction of solar wind ions reflect from an object's surface as energetic neutral atoms (ENAs). ENA emission from the Moon was first observed during commissioning of the Interstellar Boundary Explorer (IBEX) mission on Dec. 3, 2008 [ McComas et al ., 2009a]. We present the analysis of 10 additional IBEX observations of the Moon while it was illuminated by the solar wind. For the viewing geometry and energy range (〉 250 eV) of the IBEX-Hi ENA imager, we find that the spectral shape of the ENA emission from the Moon is well-represented by a linearly decreasing flux with increasing energy. The fraction of the incident solar wind ions reflected as ENAs, which is the ENA albedo and defined quantitatively as the ENA reflection coefficient R N , depends on the incident solar wind speed, ranging from ~0.2 for slow solar wind to ~0.08 for fast solar wind. The average energy per incident solar wind ion that is reflected to space is 30 eV for slow solar wind and 45 eV for fast solar wind. Once ionized, these ENAs can become pickup ions in the solar wind with a unique spectral signature that reaches 3v SW . These results apply beyond the solar system; the reflection process heats plasmas that have significant bulk flow relative to interstellar dust and cools plasmas having no net bulk flow relative to the dust.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    Sensitivity of orbital neutron measurements to the thickness and abundance of surficial lunar water (2011)
    Wiley
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    Publication Date: 2011-01-07
    Description: Recent near-infrared spectral data have shown that surficial water (H2O/OH) exists over large expanses of the lunar surface. These results have led to a reexamination of the hydrogen abundance sensitivity limits of orbital neutron data to detect surficial hydrogen on the lunar surface. A wet-over-dry, two-layer stratigraphy is modeled for the first time using neutron transport codes. For thin layers (
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 3
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    Lunar true polar wander inferred from polar hydrogen (2016)
    Springer Nature
    In: Nature
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    Publication Date: 2016-03-24
    Description: Lunar true polar wander inferred from polar hydrogen Nature 531, 7595 (2016). doi:10.1038/nature17166 Authors: M. A. Siegler, R. S. Miller, J. T. Keane, M. Laneuville, D. A. Paige, I. Matsuyama, D. J. Lawrence, A. Crotts & M. J. Poston The earliest dynamic and thermal history of the Moon is not well understood. The hydrogen content of deposits near the lunar poles may yield insight into this history, because these deposits (which are probably composed of water ice) survive only if they remain in permanent shadow. If the orientation of the Moon has changed, then the locations of the shadowed regions will also have changed. The polar hydrogen deposits have been mapped by orbiting neutron spectrometers, and their observed spatial distribution does not match the expected distribution of water ice inferred from present-day lunar temperatures. This finding is in contrast to the distribution of volatiles observed in similar thermal environments at Mercury’s poles. Here we show that polar hydrogen preserves evidence that the spin axis of the Moon has shifted: the hydrogen deposits are antipodal and displaced equally from each pole along opposite longitudes. From the direction and magnitude of the inferred reorientation, and from analysis of the moments of inertia of the Moon, we hypothesize that this change in the spin axis, known as true polar wander, was caused by a low-density thermal anomaly beneath the Procellarum region. Radiogenic heating within this region resulted in the bulk of lunar mare volcanism and altered the density structure of the Moon, changing its moments of inertia. This resulted in true polar wander consistent with the observed remnant polar hydrogen. This thermal anomaly still exists and, in part, controls the current orientation of the Moon. The Procellarum region was most geologically active early in lunar history, which implies that polar wander initiated billions of years ago and that a large portion of the measured polar hydrogen is ancient, recording early delivery of water to the inner Solar System. Our hypothesis provides an explanation for the antipodal distribution of lunar polar hydrogen, and connects polar volatiles to the geologic and geophysical evolution of the Moon and the bombardment history of the early Solar System.
    Print ISSN: 0028-0836
    Electronic ISSN: 1476-4687
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Published by Springer Nature
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  • 4
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    The effect of craters on the lunar neutron flux (2015)
    Wiley
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    Publication Date: 2015-07-16
    Description: The variation of remotely sensed neutron count rates is measured as a function of cratercentric distance using data from the Lunar Prospector Neutron Spectrometer. The count rate, stacked over many craters, peaks over the crater centre, has a minimum near the crater rim and at larger distances it increases to a mean value that is up to 1% lower than the mean count rate observed over the crater. A simple model is presented, based upon an analytical topographical profile for the stacked craters fitted to data from the Lunar Orbiter Laser Altimeter (LOLA). The effect of topography coupled with neutron beaming from the surface largely reproduces the observed count rate profiles. However, a model that better fits the observations can be found by including the additional freedom to increase the neutron emissivity of the crater area by ~0.35% relative to the unperturbed surface. It is unclear what might give rise to this effect, but it may relate to additional surface roughness in the vicinities of craters. The amplitude of the crater-related signal in the neutron count rate is small, but not too small to demand consideration when inferring water-equivalent hydrogen (WEH) weight percentages in polar permanently shaded regions (PSRs). If the small crater-wide count rate excess is concentrated into a much smaller PSR, then it can lead to a large bias in the inferred WEH weight percentage. For instance, it may increase the inferred WEH for Cabeus crater at the Moon's South Pole from ~1% to ~4%.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 5
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    Mars Odyssey neutron data: 1. Data processing and models of water-equivalent-hydrogen distribution (2011)
    Wiley
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    Publication Date: 2011-11-22
    Description: For more than 7 years, the Los Alamos built Mars Odyssey Neutron Spectrometer (MONS) has measured the neutron albedo from Mars in three consecutive energy bands: thermal, epithermal, and fast neutron ranges. This paper synthesizes the teamwork on the optimization of the signal extraction, the corrections for observational biases and instrument specific characteristics. Results are presented for neutron time series with an emphasis on seasonal variations at the poles. Frost-free data are mapped on to the surface, and the apparent random nature of the counting-rate distribution per pixel is analyzed: for epithermal neutrons, the relative standard deviation is less than 0.5% equatorward of 45° and up to 2.5% above this latitude limit; for thermal neutrons it is 1% and 2.5% respectively; and for fast neutrons it is 3% and 5.5%, respectively. New science results are obtained with regards to the distribution of water-equivalent hydrogen (WEH) on Mars. Under the assumption of a single uniform distribution of hydrogen with depth, WEH abundances range from 2% near the equator to 80% at the poles, with ±2% to 4.5% relative error bars. A best approximation to a two-layered global distribution of a lower-level hydrogen-rich substrate beneath an upper layer of varying thicknesses is generated using an average hydration level of an upper layer of 2 wt %, derived in the paper by Feldman et al. (2011). Such results are discussed and compared with regard to previous publications on the MONS instrument.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 6
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    Thorium abundances of basalt ponds in South Pole-Aitken basin: Insights into the composition and evolution of the far side lunar mantle (2011)
    Wiley
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    Publication Date: 2011-06-03
    Description: Imbrian-aged basalt ponds, located on the floor of South Pole-Aitken (SPA) basin, are used to provide constraints on the composition and evolution of the far side lunar mantle. We use forward modeling of the Lunar Prospector Gamma Ray Spectrometer thorium data, to suggest that at least five different and distinct portions of the far side lunar mantle contain little or no thorium as of the Imbrian Period. We also use spatial correlations between local thorium enhancements and nonmare material on top of the basalt ponds to support previous assertions that lower crustal materials exposed in SPA basin have elevated thorium abundances, consistent with noritic to gabbronoritic lithologies. We suggest that the lower crust on the far side of the Moon experienced multiple intrusions of thorium-rich basaltic magmas, prior to the formation of SPA basin. The fact that many of the ponds on the lunar far side have elevated titanium abundances indicates that the far side of the Moon experienced extensive fractional crystallization that likely led to the formation of a KREEP-like component. However, because the Imbrian-aged basalts contain no signs of elevated thorium, we propose that the SPA impact event triggered the transport of a KREEP-like component from the lunar far side and concentrated it on the nearside of the Moon. Because of the correlation between basaltic ponds and basins within SPA, we suggest that Imbrian-aged basaltic volcanism on the far side of the Moon was driven by basin-induced decompressional melting.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 7
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    Evidence for explosive silicic volcanism on the Moon from the extended distribution of thorium near the Compton-Belkovich Volcanic Complex (2014)
    Wiley
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    Publication Date: 2014-12-31
    Description: We reconstruct the abundance of thorium near the Compton-Belkovich Volcanic Complex on the Moon, using data from the Lunar Prospector Gamma Ray Spectrometer. We enhance the resolution via a pixon image reconstruction technique, and find that the thorium is distributed over a larger (40 km × 75 km) area than the (25 km × 35 km) high albedo region normally associated with Compton-Belkovich. Our reconstructions show that inside this region, the thorium concentration is 14–26 ppm. We also find additional thorium, spread up to 300 km eastward of the complex at ~2 ppm. The thorium must have been deposited during the formation of the volcanic complex, because subsequent lateral transport mechanisms, such as small impacts, are unable to move sufficient material. The morphology of the feature is consistent with pyroclastic dispersal and we conclude that the present distribution of thorium was likely created by the explosive eruption of silicic magma.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 8
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    Global elemental maps of the moon: the Lunar Prospector gamma-Ray spectrometer (1998)
    American Association for the Advancement of Science (AAAS)
    Details
    Publication Date: 1998-09-04
    Description: Lunar Prospector gamma-ray spectrometer spectra along with counting rate maps of thorium, potassium, and iron delineate large compositional variations over the lunar surface. Thorium and potassium are highly concentrated in and around the nearside western maria and less so in the South Pole-Aitken basin. Counting rate maps of iron gamma-rays show a surface iron distribution that is in general agreement with other measurements from Clementine and the Lunar Prospector neutron detectors.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Lawrence, D J -- Feldman, W C -- Barraclough, B L -- Binder, A B -- Elphic, R C -- Maurice, S -- Thomsen, D R -- New York, N.Y. -- Science. 1998 Sep 4;281(5382):1484-9.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Space and Atmospheric Sciences, Mail Stop D466, Los Alamos National Laboratory, Los Alamos, NM 87545, USA. djlawrence@lanl.gov〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/9727970" target="_blank"〉PubMed〈/a〉
    Keywords: *Elements ; Extraterrestrial Environment ; Iron ; *Moon ; Oxygen ; Potassium ; Spacecraft ; Spectrum Analysis ; Thorium
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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  • 9
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    Major compositional units of the moon: lunar prospector thermal and fast neutrons (1998)
    American Association for the Advancement of Science (AAAS)
    Details
    Publication Date: 1998-09-04
    Description: Global maps of thermal and fast neutron fluxes from the moon suggest three end-member compositional units. A high thermal and low fast neutron flux unit correlates with the lunar highlands and is consistent with feldspathic rocks. The South Pole-Aitken basin and a strip that surrounds the nearside maria have intermediate thermal and fast neutron flux levels, consistent with more mafic rocks. There appears to be a smooth transition between the most mafic and feldspathic compositions, which correspond to low and high surface altitudes, respectively. The maria show low thermal and high fast neutron fluxes, consistent with basaltic rocks.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Feldman -- Barraclough -- Maurice -- Elphic -- Lawrence -- Thomsen -- Binder -- New York, N.Y. -- Science. 1998 Sep 4;281(5382):1489-93.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉W. C. Feldman, B. L. Barraclough, R. C. Elphic, D. J. Lawrence, D. R. Thomsen, Los Alamos National Laboratory, MS D-466, Los Alamos NM 87545, USA. S. Maurice, Observatoire Midi-Pyrenees, 14 avenue Ed Belin, 31400 Toulouse, France. A. B. Binder, L.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/9727971" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
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  • 10
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    Fluxes of fast and epithermal neutrons from Lunar Prospector: evidence for water ice at the lunar poles (1998)
    American Association for the Advancement of Science (AAAS)
    Details
    Publication Date: 1998-09-04
    Description: Maps of epithermal- and fast-neutron fluxes measured by Lunar Prospector were used to search for deposits enriched in hydrogen at both lunar poles. Depressions in epithermal fluxes were observed close to permanently shaded areas at both poles. The peak depression at the North Pole is 4.6 percent below the average epithermal flux intensity at lower latitudes, and that at the South Pole is 3.0 percent below the low-latitude average. No measurable depression in fast neutrons is seen at either pole. These data are consistent with deposits of hydrogen in the form of water ice that are covered by as much as 40 centimeters of desiccated regolith within permanently shaded craters near both poles.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Feldman, W C -- Maurice, S -- Binder, A B -- Barraclough, B L -- Elphic, R C -- Lawrence, D J -- New York, N.Y. -- Science. 1998 Sep 4;281(5382):1496-500.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Los Alamos National Laboratory, MS D-466, Los Alamos, NM 87545, USA. wfeldman@lanl.gov〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/9727973" target="_blank"〉PubMed〈/a〉
    Keywords: Extraterrestrial Environment ; *Hydrogen ; *Ice ; *Moon ; Neutrons ; Spacecraft ; Water
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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